Sitemap for each blog subdirectory ?

Hi
Just a question :
I have two blogs installed on my root , each is a subdirectory..
Do I have to generate a sitemap for the main website and each directory like
http://www.mysite.com/sitemap.xml
http://www.mysite.com/my-blog-1/sitemap.xml
http://www.mysite.com/my-blog-2/sitemap.xml
Google sitemap generator is installed on each blog in order to be automatically generated when I post.

Thank you !